The invention relates to methods for preparing polymer films. Specifically, the invention relates to methods of preparing high biaxially oriented high density polyethylene films and the films prepared according to such methods.
Generally, in the preparation of a film from granular or pelleted polymer resin, the polymer is first extruded to provide a stream of polymer melt, and then the extruded polymer is subjected to the film-making process. Film-making typically involves a number of discrete procedural stages, including melt film formation, quenching, and windup. For a general description of these and other processes associated with film-making, see K R Osborn and W A Jenkins, Plastic Films: Technology and Packaging Applications, Technomic Pub. Co., Inc., Lancaster, Pa. (1992).
Two substantially different processes are conventionally used to orient polymeric films: blowing as a tubular film, and casting as a flat film. The two processes provide films having substantially different physical characteristics. Generally, blown films tend to have greater stiffness, toughness and barrier properties. By contrast, cast films usually have the advantages of greater film clarity and uniformity of thickness and flatness, generally permitting use of a wider range of polymers and producing a higher quality film.
An optional part of the film-making process is a procedure known as "orientation." The "orientation" of a polymer is a reference to its molecular organization, i.e., the orientation of molecules relative to each other. Similarly, the process of "orienting" a film is the process by which directionality (orientation) is imposed upon the polymeric arrangements in the film. Orientation processes are employed to impart desirable properties to films, such as making cast films tougher (i.e., increasing tensile properties).
Orientation is accomplished by heating a polymer to a temperature at or above its glass-transition temperature (T.sub.g) but below its crystalline melting point (T.sub.m), and then stretching the film quickly. The stretching imposes intermolecular alignment on the polymer. Then, on cooling, this molecular alignment competes favorably with crystallization and the drawn polymer molecules condense into a crystalline network with the crystalline domains (crystallites) aligned in the direction of the drawing force. As a general rule, the degree of orientation introduced into the polymer is proportional to the amount of stretch applied to the film, but is inversely related to the temperature at which the stretching is performed. Moreover, higher orientation also generally correlates with a higher modulus, i.e., measurably higher stiffness and strength.
When a film has been stretched in a single direction (monoaxial orientation), the resulting film exhibits great strength and stiffness along the direction of stretch, but it is weak in the other direction, i.e., across the stretch, often splitting or tearing into fibers (fibrillating) when flexed or pulled. To overcome this limitation, two-way or biaxial orientation is employed to more evenly distribute the strength qualities of the film in two directions, in which the crystallites are sheetlike rather than fibrillar. These biaxially oriented films tend to be stiffer and stronger, and also exhibit much better resistance to flexing or folding forces, leading to their greater utility in packaging applications.
It is technically quite difficult to biaxially orient films by simultaneously stretching the film in two directions. Apparatus for this purpose is known, but tends to be expensive to employ. As a result, most biaxial orientation processes use apparatus that stretches the film sequentially, first in one direction and then in the other. Again for practical reasons, typical orienting apparatus stretches the film first in the direction of the film travel, i.e., in the longitudinal or "machine direction" (MD), and then in the direction perpendicular to the machine direction, i.e., the lateral or "transverse direction" (TD).
The degree to which a film can be oriented is dependent upon the polymer from which it is made. Polypropylene, polyethylene terephthalate (PET), and nylon are highly crystalline polymers that are readily heat stabilized to form dimensionally stable films. These films are well known to be capable of being biaxially stretched to many times the dimensions in which they are originally cast (e.g., 5.times. by 8.times. or more for polypropylene).
High density polyethylene (HDPE), however, exhibits even higher crystallinity (e.g., about 80-95%) relative to polypropylene (e.g., about 70%). As a result, HDPE films are generally more difficult to biaxially orient than polypropylene films. U.S. Pat. Nos. 4,870,122 and 4,916,025 describe imbalanced biaxially oriented HDPE-containing films that are MD oriented up to about two times, and TD oriented at least six times. This method produces a film that tears relatively easily in the transverse direction. Multi-layer films prepared according to this method are also disclosed in U.S. Pat. Nos. 5,302,442, 5,500,283, and 5,527,608, which are incorporated herein by reference.
British Patent No. 1,287,527 describes high density polyethylene films that are biaxially oriented in a balanced fashion to a degree of greater than 6.5 times in both the longitudinal dimension (i.e., MD) and the lateral dimension (i.e., TD). This method requires a specific range of orientation temperatures. Also, tie layers for increasing adhesion of coatings are said to include condensation resins of an aldehyde with an interpolymer of acrylamide, a copolymer of ethylene such as ethylene vinyl acetate, or polyurethane.
U.S. Pat. Nos. 4,891,173 and 5,006,378 each disclose methods for preparing HDPE films that require cross-linking the film, with optional biaxial orientation of the cross-linked film. It is reported that the cross-linking process, which requires irradiation of the film, improves the film's physical properties. Other cross-linking processes, such as chemically-induced cross-linking, can have similar effects.
Lower crystallinity polyethylenes are typically easier to biaxially orient. For example, U.S. Pat. No. 4,680,207 describes imbalanced biaxially oriented films of linear low density polyethylene (LLDPE) oriented by being stretched up to 6-fold in the machine direction, and up to 3-fold in the transverse direction.
U.S. Pat. No. 5,241,030 describes biaxially oriented films of a blend of at least 75% of a linear ethylene/alpha-olefin copolymer, but no more than 25% HDPE. The film can be mono- or multi-layered, and can be biaxially oriented, i.e., stretched up to 8:1 in the machine direction, and up to 9:1 in the transverse direction.
U.S. Pat. No. 5,302,327 describes an anti-fogging, heat-sealable polypropylene film. The film includes a polypropylene core and a heat sealable layer of HDPE or ethylene copolymer. These bilayer films can be machine stretched up to 7.times. MD, coated or corona-treated to improve wettability, and then stretched up to 10.times. TD.
Blown films of HDPE having a ethylene-vinyl acetate heat seal coating can be used for food packaging, but such films must have a thickness of at least about 2 mil to meet the water vapor transmission rate (WVTR) requirements for packaging suitable for dry foods such as cereals. Moreover, blown HDPE films do not exhibit the dead-fold properties desirable in food packages, particularly of the bag-in-box type.
In multilaminar films, it is conventional to modify the surface properties of a substrate material to improve the adhesion between layers (interlaminar adhesion). This can be accomplished by modifying the physical or chemical properties of the substrate material itself, or by providing an intermediate layer of a material having desirable properties. Conventional intermediate "tie" or "anchor" layers are typically made from materials such as ethylene vinyl acetates (EVA), ionomers, anhydride grafted polyolefins. These materials contain polar or ionic functional groups, enhancing adhesion between polyolefins and polar polymers such as nylons, polyesters, and ethylene vinyl alcohols (EVOH). For example, the coextrusion of nylon (polar) with polyethylene (nonpolar) requires an amphipathic tie layer such as EVA having both polar and nonpolar properties in the same molecule. W A Jenkins and J P Harrington, Packaging Foods with Plastics, Technomic Pub. Co., Inc., Lancaster, Pa. (1991).
U.S. Pat. No. 5,500,283 discloses biaxially oriented HDPE films coated with polyvinylidene chloride polymers, acrylic acid polymers, and polyvinyl alcohol polymers. The coating procedure can include a priming step, e.g., chlorination, chromic acid oxidation, hot air or steam treatment, flame treatment, or high voltage corona discharge. Alternatively, such priming methods can be supplemented by adding a coating of a priming material, such as a poly(ethylenimine) material.
U.S. Pat. No. 5,527,608 discloses a multilaminar film structure including a polyolefin homopolymer or block copolymer substrate, with a heat sealable layer on one side and an HDPE layer on the opposite side. In one embodiment a metallized film is produced by corona discharge- or flame-treating a surface of the extruded polymeric structure before depositing the metal on the surface.
In view of the above considerations, it is clear that existing methods for producing biaxially oriented HDPE films yield products that are deficient in desirable physical characteristics. Existing HDPE film-making methods generally require additional chemical components in the HDPE resin (e.g., cross-linking agents) and/or additional process steps (e.g., irradiation). Such limitations complicate production, and result in increased costs. Moreover, cross-linking tends to lower polymer crystallinity, resulting in higher WVTR and lower stiffness.
Accordingly, it is one of the purposes of this invention, among others, to overcome the above limitations in the production of biaxially oriented HDPE films, by providing an economical and relatively uncomplicated method of making biaxially oriented films that imparts superior characteristics to the films, without requirement for chemical additives such as cross-linking agents, and without requirement for supplemental processing steps such as irradiation of the film.